Carey, thus, leads in both categories, while Houston has sold more physical (CD, cassette) singles than Madonna since 1991. Madonna, however, has moved a significantly higher number of digital tracks in that span than Houston, according to Nielsen SoundScan.

Billboard 200 - studio albums, soundtracks, holiday sets and greatest hits/remix collections (SoundScan 1991 to date) Since 1991, Carey has released eight studio albums, one soundtrack, one holiday set and four greatest hits/remix collections (14 total).Madonna's count stands as such: seven studio albums, two soundtracks and three greatest hits/remix collections (12 total).
And, Houston's: two studio albums, two soundtracks, one holiday set and one greatest hits/remix collections (six total).
Tallying the totals, here is the average sales number for each artist's Billboard 200 charted studio, soundtrack, holiday and greatest hits/remix efforts in the SoundScan era:

Whitney Houston: 3,308,000
Mariah Carey: 3,151,000
Madonna: 1,645,000

Here, Houston is tops - and averaging even 1.6 million in sales per albumover almost two decades is no small feat for any artist. Among the three performers, Houston's "The Bodyguard" soundtrack is the only set to pass 10 million in sales, according to Nielsen SoundScan, since 1991. The album has sold 11,808,000 copies.